This invention relates to a method of magnetic resonance (=MR) with spatial encoding to generate an image or spectroscopic data of an object of investigation inside an MR apparatus comprising the steps of:                (a) selecting a volume of interest within the object of investigation;        (b) applying an RF pulse to generate a transverse magnetization within the object of investigation;        (c) preparing a nonlinear phase distribution within the object of investigation by application of spatially encoding magnetic fields (SEMs), the SEMs comprising a nonlinear gradient field or a combination of linear and nonlinear gradient fields;        (d) effecting primary spatial encoding through application of SEMs; and        (e) recording MR signals originating from the object of investigation.        
In MR (magnetic resonance) methods, nuclear spins can be polarized by means of a strong static magnetic field and then excited and manipulated using radiofrequency (RF) pulses and controlled magnetic fields. Generally, RF pulses are delivered by means of external and transverse oscillating magnetic fields delivered in the neighborhood of the Larmor frequency of the spin.
The primary method to obtain signals from a volume of interest within an object of investigation, e.g. in magnetic resonance imaging (MRI), is by localization. There are many different methods of localization, each having particular advantages and disadvantages.
Phase scrambling can be used to localize signals by causing suppression of signals in a strong magnetic field gradient [ref. 7-12] or by performing a limited reconstruction or Fresnel reconstruction [ref. 14-15].
Also, radiofrequency (RF) coils may be used to detect signals from local regions. In this method, only spins which are in close proximity to the RF coil are detected. Other spins lying farther away from the RF coil induce less current in the RF coil and, consequently, the detectable signal from regions far from the coil is negligible.
Topical magnetic resonance is another method of signal localization [ref. 1-12], where external magnetic fields are applied to localize the signal to the homogeneous isocenter of the magnet or to the far field of an externally applied, but surface-lying gradient. These methods make use of line broadening or intravoxel dephasing techniques, such that the total signal within a detectable region is cancelled by virtue of a broad line width or uniform dispersion of spin phase over the region, respectively. This situation occurs especially in the proximal region of a strong magnetic field gradient. Localization is achieved by isolating spins which have a coherent phase and, therefore, a net detectable signal.
In previous inventions [ref. 2, 3], magnetic field gradients of 2nd order and higher are used to isolate regions of homogeneity. Gradients of 2nd order and 4th order are used to vary the size of the region of homogeneity, although one skilled in the art would recognize that combinations of high order gradient fields would give varying geometries. There is no mechanism specified for further localization of signal beyond a single lumped region or how one could move such a region or reshape it in a well-defined way. Instead, the sample itself is moved within the static field, which, unfortunately, is unsuitable for current magnetic resonance imaging techniques, which have demands on patient positioning and comfort, as well as the ease of operator use.
Other disadvantages of these techniques is that they are isolated to the homogeneous center of the magnet, broaden magnetic resonance signals unnecessarily, are poorly defined in shape, or not used as an anti-aliasing method to increase scan resolution or further localization of signal in more than one dimension (topical magnetic resonance). Apart from that, it is also difficult to manipulate or construct radiofrequency RF coils in such a way as to make a definite volume.
Slice selection uses radiofrequency pulses in combination with magnetic field gradients that may be applied to excite bands of signals that fall within the spectral response of the applied radiofrequency pulse. This method, in combination with the first, is used the most often in practice. One such method of localization in 3 dimensions uses a series of three pulses, one excitation pulse and two refocusing pulses, in conjunction with magnetic field gradients, to localize a small voxel of signal.
In more complicated and less common techniques, arrays of radiofrequency coils, with independently controllable phase and frequency, in conjunction with time-varying magnetic field gradients are used to localize signals from regions having complex shapes, such as single organs of a human body.
Some disadvantages, unique to the RF coil approach, are a) the non-uniformities of the radiofrequency field across the volume and b) the relatively long time necessary to apply the sequence of RF pulses and gradients that will localize the signal, during which the detectable signal can be lost and not recovered and the fact that c) shaped pulses used for localization may deposit too much electromagnetic radiation in human subjects.
In particular, as the Larmor frequency increases with the construction of ultra high static magnetic fields, (a) and (c) increasingly limit the durations, shapes and amplitudes of radiofrequency pulses.